The present invention relates to a park brake system for vehicles that comprises an actuator with an electric drive motor and an electronic control unit.
Conventional park brakes in vehicles usually include a pair of disk or drum brakes at the rear wheels of the vehicle, a mechanical actuating member such as a manually operated lever or a park brake pedal, and a brake cable connecting the brakes with the actuating member.
More sophisticated vehicles have an electric drive that actuates the park brake in response to a manually operated electric switch that can be mounted in the vehicle""s dash board. In such a park brake system, the park brake function is either off or on.
The present invention provides a park brake system for vehicles that permits a more flexible function.
According to the invention, a park brake system for vehicles is provided that has an actuator with an electric drive motor and an electronic control unit provided with an interface. A reduction gear has an input connected to the output of the electric motor and an output member for connection to mechanical brakes of the vehicle. A command unit has a transducer connected to the electronic control unit. The transducer converts position indication signals into appropriate digital signals to enable the control unit to generate corresponding brake control commands. With appropriate control signals from the electronic control unit, the mechanical brake system can thus reflect positional brake commands from the command unit within an operating range from OFF to ON, including partial activation conditions of the park brake.
A park brake typically requires high actuation forces above 1000 N. Generating sufficiently high actuation forces with an electric drive normally entails high consumption of electric power. In addition, when the park brake is in the activated condition, it must remain in that condition even after ignition has been switched off and, therefore, without consumption of electric energy. Conventionally, some mechanical blocking means such as a latch would thus be required to maintain the park brake in an activated condition independent of the availability of electric current.
A preferred embodiment has a high efficiency drive with an inherent blocking function so that a fast operation at moderate electric power consumption is ensured without the need for a separate blocking mechanism. According to this embodiment of the invention, the reduction gear comprises a fist reduction train and a second reduction train, the first reduction train including a toothed belt connecting an output gear of the drive motor with an intermediate gear and the second reduction train including a threaded spindle and a screw nut engaged with the spindle, the second reduction train being functionally arranged between the intermediate gear and the output member. By selecting an appropriate geometry for the screw thread of the spindle, a self-locking feature is achieved, i.e. transmission is possible in only one direction (from input to output).
Another advantageous embodiment of the invention provides an haptic feedback to the driver of a manual park brake actuation process. According to this embodiment, the command unit comprises a manual actuating member movable within a predefined actuating range extending between a home position and an end position, any actuating stroke of the manual actuating member within this range being converted by the transducer into brake control commands adapted to move the output member across a corresponding stroke within a predefined range of brake strokes. Preferably, the command unit has a ratchet mechanism with a predetermined number of latching positions along the actuating range for selectively latching the actuating member in one of the latching positions, and further has a spring biasing the actuation member to the home position. The ratchet mechanism would typically provide a clicking sound in operation, similar to the sound generated by a conventional park brake lever.